The present invention relates to methods and apparatus for bonding a silicon die to a package in the construction of a semiconductor chip. Conventional die attach bonding methods include gold/silicon die attach bonding. Since in Au/Si die attach bonding the die is attached to the package at temperatures above 400.degree. C., the Au/Si die attach method is suitable for bonding silicon dies to ceramic rather than plastic packages. One limitation of the prior art gold/silicon die attach bonding methods is that these methods must use a die of the size 62.5K mil.sup.2 or smaller in order to meet the die attach bond's thermal resistance requirements. In the prior art method, the preform is unevenly heated and cooled and the die bond of a large die cools too quickly. Therefore, when the prior art method is used for a large die, stresses and voids are formed in the die bond and the die bond has a high thermal resistance.
The prior art Au/Si die attach method uses an Au/Si preform to attach the die to the package. The package cavity is heated up to a temperature above the Au/Si eutectic point so that the Au/Si preform will melt. The preform is put into the cavity so that it melts. Then, the die is placed in the package cavity and pressed down on the preform and package so that the die attach bond is made in a process called scratching. After the die attach bond is made, the bond is cooled through natural heat dissipation during which no heat is supplied to the die attach bond.
U.S. Pat. Nos. 4,771,018 and 4,810,671 disclose an Au/Si die attach bond method that uses an Au/Si seed of a size that is approximately ten to twenty percent (10-20%) of the surface area of the die, in addition to using an Au preform attached to the package cavity.
Another conventional die attach method is the silver/glass die attach. Silver/glass die attach can attach dies of larger sizes than the prior art Au/Si die attach methods. However, the silver/glass die attach method takes at least three hours for a pre- and post-curing process to meet the thermal resistance and mechanical strength requirements of a die with 176K mil.sup.2 area. The thermal resistance, .theta..sub.jc, has been determined to be 0.9.degree. C./W in the silver glass die attach method for a 176K mil die, where .theta..sub.jc is defined by the equation: ##EQU1## P is the power put into the device; T.sub.j is the temperature at the junction surface of the die which is defined as the surface of the die where the circuit is located; T.sub.c is the temperature at the case. The case is the bottom surface of the package when the package is oriented to be attached to a board ("live bug" situation). A low thermal resistance is desired because a die attach bond with a low thermal resistance is able to more effectively dissipate heat from the operating die.
It is an object of the present invention to have an Au/Si die attach method that can be used with large dies; as the term is used in this application, large dies refer to dies with a size above 62.5K mil.sup.2. Another object of the present invention is to have a die attach method that has a low thermal resistance, and a substantially void-free die attach bond. Voids in the die attach bond causes a poor thermal resistance, .theta..sub.jc, and may cause stress fractures.